An insulated-gated field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located within a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
To electrically isolate IGFETs or other devices fabricated within the same silicon substrate, techniques such as LOCOS and SWAMI are used. LOCOS is the local oxidation of silicon, a surface isolation scheme where silicon dioxide is grown around islands of silicon nitride, which is in turn removed to leave oxide-free areas for the formation of circuit components. SWAMI is an enhanced LOCOS-like technique.
Still another technique is the formation of a trench isolation, and more particularly the formation of a shallow trench isolation, within the substrate. The procedure for forming a trench insolation is the same as forming trench capacitors. Thus, trenches are etched either isotropically with wet techniques or anistropically with dry etch techniques. An insulating material, such as silicon dioxide, may then be deposited within the trench to act as a insulating layer.
A difficulty with trench isolation, however, is that the formation of subsequent metallization layers may result in overetching of the insulating material (e.g., silicon dioxide), such that deposition of a metal shorts an active region with the silicon substrate, which is usually grounded. For example, one metallization technique involves first applying a thin layer of silicon oxynitride prior to the deposition of tetra-ethyl-ortho-silicate (TEOS). The silicon oxynitride acts as an etch stop when the TEOS is selectively etched to expose the components to be, for example, locally interconnected (LI'ed).
Because silicon oxynitride is an insulator, however, it must be removed prior to depositing metal in the area exposed by the selective etching of the TEOS. Etching the silicon oxynitride is usually not selective to silicon oxynitride, however; silicon dioxide may also be etched. Thus, the oxide material within a trench may be etched away during removal of the silicon oxynitride layer, exposing the silicon substrate such that a short between an active region and the substrate may result.
There is a need, therefore, for prevention of such shorting during metallization (e.g., during the formation of local interconnects.